In an air jet weaving system, compressed air is used to draw weft threads from supply spools and to blow them into the shed of an air jet loom. A set of relay nozzles is used to support the movement of the weft thread across the shed, which may be several meters in width. Additional nozzles at the far end of the shed may stretch an inserted thread during a weaving operation. An example air jet loom is described in U.S. Pat. No. 4,534,387.
It is known to adjust the airflow to the main or to the relay nozzles according to the kind of weft thread to be woven. For example, a smooth and strong filament yarn can be woven with a high airflow at the relay nozzles while a weak spun yarn, or a spun yarn with several irregularities, can be woven only with a lower airflow at the relay nozzles. In order to successively insert two or more kinds of weft threads, the airflow of the relay nozzles can be set at a value required by the weakest type of weft yarn so that the weft yarn is not blown apart, broken or damaged.
U.S. Pat. No. 4,534,387 provides two airflow rates for the relay nozzles and selects the correct airflow such that a yarn will not be blown apart and such that the yarn will be inserted across the shed to arrive timely at the other side or far end of the shed. The weaving machine speed is adapted to suit to the slowest yarn. This machine requires a separate pressure-reducing valve for each required airflow rate.
U.S. Pat. No. 6,062,273 describes an electrically actuated throttle valve for an insertion nozzle. The throttle valve comprises a plunger which is movable, in a linear direction, within a bore hole. The plunger can be positioned at a desired distance from a valve seat. This type of valve is relatively slow to operate. In situations where the airflow rate needs to be varied for each insertion, the valve has to operate in a period of less than 35 msec. A second problem with this type of valve is that it has a rubber sealing ring which surrounds the plunger to prevent compressed air from escaping from the valve. This sealing ring is prone to wear and thus this type of valve has a limited life time when used in situations where the airflow rate needs to be varied for each insertion.
It is desirable to have a reduced number of valves as these valves are expensive and are volume/area consuming items. It is also desirable to provide a valve which has a long life time and which does not wear so rapidly. Further, it is also desirable to have a rapidly operating valve.